Resinous block copolymers have been produced by methods employing various sequential polymerization steps. Among the pioneer inventions in the field of resinous block copolymers are such patents as U.S. Pat. No. 3,639,517 to Kitchen and Szalla, U.S. Pat. No. 4,080,407 to Fodor, and U.S. Pat. No. 4,091,053 to Kitchen.
Much effort has been directed to the preparation of substantially transparent block copolymer resins with a variety of block structures produced by a variety of monomer addition sequences.
However, products frequently have had residual coloration tinges, or the impact strength has been low, or importantly for many purposes there has been a high incidence of crazing (flexural stress clouding).
One of the more important applications for transparent resinous copolymers has been in blister packaging of bandages, syringes, and the like for contained product protection and maintenance of sterile conditions during shipping. Unfortunately, all too frequently, crazing of the blister packs has occurred because of squeezing during shipping. The crazed packs, plus their valuable contents, then must be discarded by the recipients, such as hospitals or physicians, since it is presumed that air has leaked into the pack and that the contents have lost their sterility due to contamination.
Other usages of the transparent resinous polymers have been for blending with general purpose polystyrene. The transparent resinous polymers have been used for various molding purposes. However, many of the so-formed products have had a bluish tinge, or lacked clarity because of haze, frequently leading to customer objection and reluctance to purchase.
Needed, still, are resinous copolymers, readily made, impact resistant, exhibiting a high resistance to crazing, transparent with good clarity, substantially no color, and substantially haze-free.